1. Field of the Invention
The present invention relates generally to an adaptive antenna system and in particular, to an efficient radio resource allocating method and apparatus for improving a system throughput with respect to a scheduled user.
2. Description of the Related Art
The Institute of Electrical Electronics Engineers (IEEE) 802.16 wireless Metropolitan Area Network (MAN) standard is standardized to use a Space Division Multiple Access (SDMA) scheme in an adaptive antenna system. The SDMA scheme allocates a radio resource positioned in the same frequency and at the same time point to a plurality of Mobile Stations (MSs) simultaneously using a directional antenna capable of forming a beam. The SDMA scheme is a radio communication technology that can drastically increase the system capacity when there are multiple MSs.
FIG. 1 is a diagram of a Downlink (DL) frame structure in a general Adaptive Antenna System (AAS). The DL frame of FIG. 1 includes a preamble 100, a non-AAS area, and an AAS area. The AAS area enables the SDMA with respect to the radio resource. Hence, a radio resource corresponding to AAS traffic #0 110 can be allocated to a plurality of MSs 111, 113 and 115 at the same time. Radio resources corresponding to AAS traffic #1 120 and AAS traffic #2 130 can be allocated to the plurality of MSs at the same time.
As such, when the SDMA scheme is adopted in the adaptive antenna system, a Base Station (BS) can overlappingly allocate the radio resource positioned in the same frequency and at the same time point to one or more MSs. In this case in the beam formation correlation may occur between the MSs assigned the overlapping radio resources. The correlation may make it difficult for the MSs to successively extract their received signal. To prevent this, the BS selectively allocates the radio resources of the same position to MSs that suffer less correlation, using a scheduling algorithm so as to maintain the independence between the radio spatial resources.
In the meantime, an amount of data carried by the allocated spatial resource is determined by a user channel quality and a transmit power allocated to the spatial resource. The user channel quality varies according to time and space whereas the transmit power is fixed by the system. Hence, users using the radio resource at the same position need to properly divide and utilize the fixed transmit power. In doing so, when the user is allocated more transmit power, the channel receive sensitivity of the corresponding user increases and more data is delivered by the allocated resource. By contrast, when the user is allocated less transmit power, the channel receive sensitivity of the corresponding user decreases and less data is delivered by the same resource.
FIG. 2 is a diagram of a scheduling and resource allocating method in the general adaptive antenna system. Typically, a scheduler sequentially allocates radio resources according to priority of users using user information stored to a queue management module. Specifically, the scheduler allocates spatial resources for Spatial Frames (SFs) #1, #2, and #3 corresponding to a radio resource Resource #1 by taking into account the scheduling priority and the correlation between the users. Upon completing the allocation, a radio resource for a Resource #2 is allocated in the same manner and the allocation is repeated up to a Resource #N.
The conventional scheduler allocates the radio spatial resources to maximize the system throughput merely by taking into account a Modulation order Product code Rate (MPR), without considering the user channel quality and the allocated power. FIG. 3 illustrates drawbacks of the conventional scheduling and resource allocating method. Table 1 is a Carrier to Interference and Noise Ratio (CINR) and MPR table used in the example of FIG. 3. The MPR is a ratio of a transmittable information amount using the radio resource of the same size. The MPR is classified into a plurality of levels. Each level corresponds to a CINR value and determines a Modulation and Coding Scheme (MCS) level according to the MPR level.
TABLE 1CINR (dB)MPRMCS level20.17QPSK ½ repetition 630.25QPSK ½ repetition 450.5QPSK ½ repetition 281.0QPSK ½ repetition 1111.5QPSK ⅔142.016QAM ½173.016QAM ¾204.016QAM ⅔245.064QAM ⅚
In FIG. 3a, when the allocation up on SF #3 is finished, the sum of MPR values is 5.5 (=3.0±2.0±0.5). Hence, when all of SF #1, #2 and #3 are allocated, the system throughput is most excellent. However, in a case of the allocation up to SF #2 in FIG. 3b, the sum of MPR values is 5.0, which is better than the sum 4.5 (=2.0±2.0±0.5) of MPR values in the allocation up to SF #3. In this case, the system throughput becomes better when the spatial resources are allocated to only two users, rather than three users. In FIG. 3c, when only SF #1 is allocated; that is, when the spatial resource is allocated to a sole user, MPR 4.0 exhibits the best system throughput. In other words, when the scheduler allocates the radio spatial resources by taking into account the MPR alone, without considering the user channel quality and the allocated power, the spatial resources are allocated to the sole user. As a result, the radio spatial resource utilization may deteriorate.
Therefore, what is needed is a radio resource allocating method for maximizing the system throughput by taking into account the user channel quality and the allocated power.